ALPM

Aggressive Link Power Management (ALPM) is a mechanism where a SATAAHCI controller can put the SATA link that connects to the disk into a very low power mode during periods of zero I/O activity and into an active power state when work needs to be done. Tests show that this can save around 0.5-1.5 Watts of power on a typical system.

ALPM is now available in several SATA controllers that use the Advanced Host Controller Interface (AHCI). However, there is some anecdotal evidence that some controllers may go into a low power state incorrectly and this ends up causing data loss. Ubuntu has the ability to use ALPM but it it disabled by default since it can cause data loss on some machines.

We are looking for volunteers to test ALPM for 2 reasons:

help to identify typical power savings on a range of machines

help to identify chipsets (and machines) where ALPM works reliably and also where it is broken and needs fixing.

NOTE: There is a risk in doing these tests. There is a possibility that ALPM may aggressively power down the SATA link prematurely or incorrectly and cause data corruption or loss. YOU HAVE BEEN WARNED!

How to Test

Gather the BIOS version (so that we can differentiate between subtle versions of the same machine) using:

sudo dmidecode -t 0 | grep "Version:"

Gather the kernel version:

uname -a

Gather the SATA AHCI controller information using:

lspci -vn | grep AHCI

..if you do not have an SATA controller that uses AHCI then don't proceed with this test.

Unplug the power from your laptop/netbook and run the test on battery power taking note of the average power consumption and standard deviation - these are shown at the end of the powerstat run.

sudo pm-powersave true
powerstat

Enable ALPM:

echo SATA_ALPM_ENABLE=true | sudo tee /etc/pm/config.d/sata_alpm

If you want to turn ALPM on and off depending on power state, add the following two lines to /etc/pm/config.d/sata_alpm (not needed on Ubuntu 14.04):

SATA_LINKPWR_ON_AC=max_performance
SATA_LINKPWR_ON_BAT=min_power

Re-test using powerstat again:

sudo pm-powersave true
powerstat

And perhaps exercise you machine (e.g. use it for 24 hours with ALPM enabled) to see if ALPM works correctly without corrupting any data.

If required, remove the ALPM enable setting:

sudo rm /etc/pm/config.d/sata_alpm

Add your findings to the following table (please put name of the machine in sorted alphabetical order). For the power measurements enter the average power consumed and standard deviation data for both test runs.